82 The. American Geoloaisf. February. 18% 
gestive and whicli ni;iy be prolitable in directing; future 
inquiry. 
Before proceeding to the subject prc>per a few i-euiarks will 
be in place concerning the classification of tlie rocks and the 
distribution of formations. In the following table is present- 
ed a scheme which is the outcome of recent work by the writer 
and associates of the State Geological Survey. 
The names and the divisions of this scheme dillerin several 
respects from those previously published. We will not attempt 
here to give the reasons for these changes. They will appear 
in the forthcoming report of the Geological Survey on the 
lead and zinc deposits of the state, now being printed. We 
shall use this classification and nomenclature in the following 
discussion. 
Distribution. — The Archean rocks of Missouri occur exclu- 
sively in the southeastern part of the state. They are confined 
principally to an area of about 2,000 square miles, situated 
less than 80 miles south of St. Louis and less than 50 miles 
west of tile Mississippi river. They consist essentially of 
porphyries and granites, composing a group of hills named 
the St. Francois mountains. 
The Lower Silurian (and possible Cambrian) rocks of the 
Ozark stage surround these Archean crystallines on all sides. 
They extend eastward nearly to the Mississippi river, north- 
ward to the Missouri, excepting in the vicinity of St. Louis, 
westward to within 50 milesof the Kansas line, and southward 
beyond the border of the state. The^^ consist principally of 
magnesian limestones and sandstones. 
The Trenton and liigher Lower Silurian formations are seen 
to overlie the Ozark formation only over a small area south of 
St. Louis and along the Mississippi river. Over the whole 
western portion of the state they are absent. The}' consist 
mainly of magnesian limestones. 
Upper Silurian strata are aljsent in the state, with the ex- 
ception of a few exposures in the east, adjacent to the Missis- 
sippi. 
Devonian rocks are sparsely represented in the eastern and 
northeastern portions of the state. Neither the thickness or 
area is anywhere great. A few isolated patches are found 
along the Missouri river, on the north side. In the west the 
